rancorous

The adjective rancorous comes in handy when you're describing someone's bitter grudge, like the attitude of your cousin who still won't speak to you after the April Fool's Day prank you played four years ago.

A story can be rancorous, if it's full of resentment, and so can an argument, if it's particularly bitter and angry. The word rancorous can be traced back to the Latin word rancere, which means "to stink." This in turn led to rancorem, "bitterness or rancidness." When you speak to your arch enemy and your words are so angry and bitter that they almost stink, go ahead and describe them as rancorous.